OK, but this isn't a question of the organising body keeping the original trophy (which as you say, is quite normal). This is a situation where someone who has no business doing so - who has no connection to the game of football or the competition (apart from happening to show up for the final) getting to keep the original trophy, for no better reason, as far as I can tell, than his liking for shiny, gold-coloured things.
According to Trump, Infantino has already told him they don't want the trophy back (and he apparently also took one of the winners' medals).
Donald Trump takes a Club World Cup winners' MEDAL - and he's keeping the trophy in the Oval Office